Improve photo loading and full-screen gallery swiping

[Xare123]

Summary

• Guard asynchronous image loads from updating disposed widgets
• Bound decoded image dimensions to the displayed size and a 1024-pixel ceiling
• Let unzoomed full-screen photos yield horizontal gestures to the surrounding gallery
• Add deterministic coverage for normal, invalid-metadata, and oversized decode dimensions

Why

Large photos should not be decoded at unnecessarily high resolution for a small message bubble, and users should be able to swipe directly between full-screen images. Zoomed photos retain their own pan behavior.

Validation

• flutter test test/app/layouts/conversation_view/widgets/message/attachment/image_viewer_decode_dimensions_test.dart --reporter expanded
• 3 of 3 focused tests passed
• Combined validation with Retry transient sends and shared-profile downloads #250 and Prioritize requested attachment downloads #252: 16 of 16 available tests passed
• Targeted analysis found no introduced errors
• git diff --check passed
• Hardened combined CI passed the complete Dart suite, real Android APK build, required native-library assertions, and artifact upload

Manual interaction still recommended

Open a thread with multiple photos, swipe between them, zoom and pan without accidental paging, then dismiss and reopen while an image is loading.

Scope

This intentionally excludes the broad media cache, audio/video controller, sticker, reaction, lifecycle, workflow, and documentation changes from #231.